The transmission of data in digital form is becoming common place. Present day transmission channels over which digital data is sent suffer from various impairments including additive impulse noise.
Additive impulse noise can disturb the operation of various systems within a receiver such as the demodulator portion of the receiver. The demodulator portion of a receiver can include, e.g., a carrier recovery system and an equalization update system. Such systems can have difficulty in performing their functions properly when highly erroneous impulse noise signals are present.
The Electronic Engineers' Handbook, third edition, McGraw-Hill 1989 states as a general proposition for receiver circuits that "if the noise consists of large impulses of relatively low density, the best system performance is obtained if the system is limited or blanked during the noisy periods and behaves normally when the (impulse) noise is not present."
While this statement suggests the desirability of limiting or blanking a receiver during the receipt of impulse noise, it fails to describe how to detect the presence of impulse noise in a signal representing digital data and how to implement the limiting or blanking operation suggested.
In the case of a signal representing digital data, the signal may be demodulated to correspond to one of a plurality of different discrete energy levels, impulse noise affecting the decision processes may fall within the overall permitted amplitude range for the signal being transmitted but still result in improper decisions being made since the impulse noise can cause a signal to take on an erroneous or indeterminate value. This may affect a circuit's ability to accurately demodulate received symbols over a period extending beyond the duration of the impulse noise.
Because of carrier recovery implications, improper decisions resulting from impulse noise can present particularly serious problems in systems which use Quadrature Amplitude Modulation ("QAM"). In many QAM demodulator circuits carrier recovery is performed by calculating a phase error on each received symbol based upon the angular difference between an incoming symbol and its target, e.g., desired, value. This phase error is then used to drive a phase-locked loop (PLL), e.g., a digital PLL (DPLL), to lock a derotating sine wave to the signal. When noise impulses arrive, the phase error determination can be totally wrong if the incoming symbol is driven into an incorrect decision box. In such a case, an incorrect target symbol may be used in the phase error calculation resulting in a possible loss of signal lock which can affect the decoding of several symbols thereafter.
Accordingly, there is a need for providing a method and apparatus for accurately determining the presence of impulse noise in a digital receiver circuit, such as a QAM or PAM-VSB demodulator circuit. Furthermore, it is desirable that such methods and apparatus be able to determine the presence of impulse noise even when other noise is relatively small thereby permitting proper limiting or disablement of various portions of the receiver system during the presence of impulse noise to prevent, e.g., loss of signal lock.
Furthermore, it is highly desirable for economic reasons that such impulse noise detection methods and apparatus be relatively simple, easy to implement, and compatible with a plurality of demodulator designs.